Giardia lamblia, a flagellated protozoan and common cause of gastroenteritis, is a rare but previously reported cause of reactive arthritis (ReA). We report a case of inflammatory oligoarthritis in a young woman after infection with Giardia. Two weeks after being treated, she developed an inflammatory arthritis of her left knee and right elbow that was refractory to nonsteroidal antiinflammatory medication. Antinuclear antibody, rheumatoid factor, and HLA-B27 tests were negative. She had almost immediate relief with intraarticular injection of corticosteroids. We review the previously reported cases of ReA following giardiasis and discuss possible pathogenic mechanisms. Although ReA most commonly occurs after chlamydial urethritis or gastroenteritis associated with typical enteropathic bacteria, important historical clues could point to less common pathogens such as Giardia. Physicians should be aware of these less common causes of ReA, because this could have important diagnostic and therapeutic implications.